H-1 NUCLEAR-MAGNETIC-RESONANCE AND MOLECULAR-ORBITAL STUDIES OF THE INTERNAL ROTATIONAL POTENTIAL AND ELECTRON DELOCALIZATION IN TRIPHENYLPHOSPHINE

The H-1 nuclear magnetic resonance spectral parameters are reported fo r triphenylphosphine as solutions in CS2/C6D12 and acetone-d6 at 300 K . The internal rotational potential opposing torsion about the P-C bon d is computed by AM1 and STO-3G MO methods. The computed potentials ar e used to calculate die average shielding of die para protons caused b y the diamagnetic anisotropies of the neighbouring phenyl groups. The results are used to estimate the degree of electron delocalization fro m the lone pair on phosphorus. The extent of delocalization depends on the internal motions and comparisons are made with phenylphosphine. T he maximum possible screening of the para protons in phenylphosphine i s calculated as 0.19 ppm for a conformation in which the lone pair on phosphorus is oriented perpendicular to the aromatic plane. The intram olecular rotational potentials then yield 0.02(9) ppm as the shielding of the para protons in triphenylphosphine due to electron delocalizat ion, just as found for the CS2/C6D12 solution after correction for dia magnetic anisotropy fields.